Vacuum gauge



R. H. VARIAN March 28, 1950 VACUUM GAUGE Filed March 31. 1945 dej ml N..WWE

INVENTOR RUSSEL/ H. l//w/F/N f ATTORNEY VPatented Mar. 28, 1950 UNITEDSTATES PATENT UFFICE VACUUM GAUGE Russell H. Varian, Garden City, N. Y.,assigner to The Sperry Corporation, a corporation of Dela- ApplicationMarch 31', 1945, Serial No. 586,006

9 claims. (ci. 175-183) The present invention relates to methods andapparatus for measuring very low gas pressures such as are encounteredin evacuated electron discharge devices, usually referred to as vacuumtubes.

The principal object of the present invention is to measure the gaspressure within an evacuated space.

It is another object oi this invention to provide a method ofdetermining the gas pressure in a vacuum tube. l

Another object is to provide -an improved vacuum gauge.

It is a further object to provide a vacuum gauge adapted for measuringgas pressures throughout a relatively extensive range such as the rangefrom 10e to 10-10 millimeter of mercury.

In the present invention, electrons are projected along a directive beamthrough an evacuated space, and the compactness of the electron streamis modulated by the formation of ions therein. The number of ions withinthe electron beam tends to remain substantially equal to the number ofelectrons in the beam, and thus does not vary appreciably. yThis isseenfrom the fact at any instant in the beam, the beam tends to have anet positive charge which, by repelling the ions from the beam,increases the rate at l which theions drift from the beam. Thus, the

ion content of the beam is-self-ad-.iusting to a number of ions justsuilicient 'to neutralize'the beam charge. The rate of formation ofvionsvin the electronbeam by collision of electrons with gas molecules isapproximately proportional to the gas pressure. Accordingly, as the gaspressure is decreased, and the rate roi? ion formation correspondinglydecreased, each ion must remain effective in` the electron beam longerthan under higher gas pressure conditions, in order to satisfy the aboverequirement that the number of ions within the electron beam remainsubstantially equal to the number of electrons short. Under a conditionof very low gas pres-4 sure, new ions are formed infrequently, and thetime span of effectiveness of an ion in the beam is very long.

If no ions were present within Y the space through which a beam ofelectrons is projected, the strong negative charge of the beam wouldtend to cause dispersion of the electrons. The presence of ions withinthis space tends to neutralize the negative charge, eliminating theelectron-repellent tendencies and permitting a compact electron beam tobe produced. The ions disappear from the beam and new ions arecontinually formed to replace them. Since the formation of a new ion isgenerally not simultaneous with the cessation of eilectiveness ofanother ion, it will be recognized that the number of ions eiective inthe beam does not remain exactly uniform, but instead increasesmomentarily at the instant of each ion formation.

The amount of dispersion of the electrons from an ideally compactelectron beam varies appreciably with a reduction or an increase by oneion in the number of ions effective in the electron beam. Accordingly,the compactness of the electron beam is modulated in accordance with theformation of new ions in the beam, the frequency of modulation beingvaried accordingly as the ion formation is frequent with high gaspressure, or infrequent with low gas pressure.

By providing an electron lcollector adapted to receive only thoseelectrons which are within a predetermined very short distance from theaxis y of the electron beam, there is produced an electric current whichis modulatedv in accordance withthe compactness modulation of theelectron beam. The modulationv of this current corresponding to a givengas pressure is not 'a simple periodic wave,since.new ions are' notformed in the electron beamv at precise, regular intervals. On the otherhand. the modulation is characterized bv a distribution of energythrough a wide range of frequencies with maximum amplitude at afrequency dependent upon the pressure of the gas. Due to inertia oi theions, and to the tendency of all ions in the beam to be repelled whenthere is excess positive charge. very low- Thus, by analyzing theelectron collector cur- 3 rent in terms of the frequency distribution ofenergy, the gas pressure in the evacuated space may be ascertained.Since the frequency distribution of energy varies as a predictablefunction of gas pressure throughout a range extending far beyond therange of gas pressures to which ordinary vacuumv gauges are limited, thepresent invention provides an absolute measurement of pressure inextremely high-vacuum devices.

Apparatus suitable for use in determination of the gas pressure withinan evacuated system is illustrated in Fig. 1, while Fig. 2 illustratesthe frequency distribution of output noise in the apparatus of Fig. 1resulting from each of three different gas pressures Within the system.

Referring now to Fig. 1, an electron discharge device II, including acylindrical metallic chamber I2, an electron gun I3 and an electroncollector end I4, is connected as by a tubular conduit I5 in a systemwhich may include a vacuum tube in the process of being evacuated.

The electron gun I3 comprises an insulating gas-tight supporting andenclosing portion such as a cup-shaped glass portion I6, which may besealed to a tubular metal member I1 connected in turn to one end 36 ofthe cylindrical chamber I2. Within the glass portion I6 is provided anelectron emitter or cathode I8 arranged symmetrically about the axis ofthe cylindrical chamber I2,

and supported by a rigid connection to a stiff :z:

terminal wire I9 extending through a seal 2| in the glass portion I6. Asecond conductor 22 similarly extending through the glass portion I6 maybe connected to one end of a heater winding 23 having its opposite endconnected to the wire I9.

A focusing electrode 24 is connected to vcathode I8 and supported inspecially positioned relation therewith and arranged to direct electronsemitted by cathode I8 into a concentrated stream or electron beam alongthe axis of the cylindrical chamber I2, the cross-section of this streambecoming very small at the portion thereof approaching the end 31 of thecylindrical chamber I2.

The electron collector end I4 may include a tubular metal portion 3lconnected to the end 31 of the chamber I2, and a cup-shaped glassportion.32 sealed to the tubular portion 3|. A collector electrode 33 issupported within the collector end by a further sti terminal wire 34,sealed as shown at 35 in the glass portion 32.

Each of the disc ends 36 and 31 of the cylindrical chamber I2 isprovided with an opening symmetrical about the axis of the cylindricalchamber I2. The opening through the end 36 may be of a diametersubstantially equal to or greater than the diameter of cathode I8 forpermitting the electron beam produced by the cathode I8 and the focusingelectrode 2i to enter the cylindrical chamber I2. The opening throughthe opposite end 31 of the chamber I2 preferably, but not necessarily,may be made appreciably smaller than the input opening, as illustratedin Fig. l, so that the relative extent to which the electron beam isdirected through the exit opening in the end 31 will depend greatly uponthecompactness of the electron beam approaching this opening. Ifdesired, an electron-permeable grid 38, which may comprise a screen ofsmall metal wires or a perforated disc, for example, may be insertedwithin the opening in the end 36 of the chamber i2, and a similar grid39 may be inserted within the electron exit opening in the end 31.

A rst electric energy source such as a battery III is connected throughthe terminal wires I9 and 22 to the heater winding 23 of the electrongun I3, for heating theY cathode I8 to provide electron emissiontherefrom.

An electron accelerating potential source such as a battery 42 isconnected at its negative terminal to the cathode I8 and at its positiveterminal to the chamberV I2 for accelerating to an apreciable velocitythev electrons in the beam issuing from the electron gun I3. The source42 may provide an accelerating potential of the order of 500 voltsbetween the cathode I8 and the end 35 of the cylindrical chamber I2. Theheated cathode for emitting electrons, the focusing electrode 24', andthe polarized end 36 of the chamber I2 cooperate to project the beam ofelectrons through chamber I2 toward the exit opening in the end 31thereof.

If desired, the cylindrical chamber I2 may be grounded, as illustratedin Fig. 1 at 43.

A collector polarizing source such as a battery 44 is .provided formaintaining the collector electrode 33 at a relatively small positivepotential with respect to the cylindrical chamber I2 and ground, apotential of the order of 25 volts being adequate for this purpose. Thenegative terminal of the battery 44 is connected to the cylindricalchamber I2 and to ground at a junction 45. and its positive terminal isconnected through an impedance such as a resistor 46 to the collectorelectrode 33.

A wave analyzer 41 arranged to provide a meas` urement of alternatingenergy at any frequency selected by adjustment of a variable frequencycontrol is coupled at one termin al to the collector electrode 33 of thedevice I I by a coupling capacitor 48, the other input terminal of thewave analyzer being connected to junction 45. The wave analyzer 41 maybe such an instrument as the General radio wave analyzer, model 'B6-A.Alternatively, it may be such an instrument as is shown in U. S. Patent1,994,232, issued March 12, 1945, to O; H. Schuck, Jr., for Waveanalyzer.

In the operation of the electron discharge device illustrated in Fig. 1,the cathode I8, which may have a planar or slightly concaveelectronemitting surface is heated by the heater electrode 23 and ismade to emit electrons, which are attracted toward the electron entranceopening in the wall 36 to the chamber I2. The focusing electrode 24cooperates with the cathode I8 to produce predetermined voltagegradients in the vicinity of the cathode I8 such that the electronsemitted thereby are projected in a concentrated electron beam or streamalong a predetermined axis, which may be the axis of the cylindricalchamber I2.

Those electrons in the beam projected through the chamber I2 which arenear the axis of the chamber are permitted to pass through the exitopening 31 and to be received by the collector electrode 33. On theother hand, those electrons which are spaced appreciably from the axisof the chamber I2 are arrested by the end wall 31 of the device, andthus are prevented from' being received by the collector electrode 33.

The electric current which circulates through the cathode I8, thecollector electrode 33, the impedance 46, and energy sources orbatteries 4d and 42, thus increases or decreases accordingly as theelectrons in the beam projected by the electron gun I3 are closelyconcentrated or scattered in the region near the exit through the Wall31 of the chamber I2.

Since as pointed out above, the scattering of the electrons from theaxis of the electron beam is sharply dependent upon the number oi.'positive ions in the beam, and since slight variations in the number ofpositive lons in the beam occur at a frequency substantiallyproportional to the gas pressure therein, the frequency distribution ofvariations in the beam scattering is indicative of the gas pressurewithin the space through which the electron beam is projected.Accordingly, the variations in the electric current through4 thecollector electrode 33, and thus of the voltage produced across theimpedance I6, may be frequency analyzed to determine the gas pressure inthe device Il, and thus the gas pressure in the system in which thedevice ii is connected by conduit I5.

In Fig. 2 is shown a graph including a family of three curves, each ofwhich represents the distribution of electrical output energy vs.frequency corresponding to a selected pressure of the gas within thedevice I I. Curve I, representing a relatively poor vacuum; or high gaspressure, has its maximum energy content at a relatively high audiofrequency fi, and has lower intensity at frequencies above and below f1.Curve II, representing a better vacuum, which might be characterized bya gas pressure of the order of '1 millimeter of mercury, for example,contains maximum energy at an appreciably lower frequency f2, againhaving a wide range of frequency components above and below this valueat which the output is appreciably reduced. Curve IH illustrates anenergy vs. frequency distribution having a peak at an extremely lowfre-Y quency fa, corresponding to an extremely high vacuum, as., forexample, a gas pressure of the order of 10-9 millimeter of mercury. Thefrequency of the maximum alternating current energy derived from theelectron beam is substantially proportional to the gas pressure.

In the apparatus shown in Fig. 1, the substantially unvarying totalelectron current in the beam is divided into two parts, the rst partbeing a selected middle or central portion of the electron stream whichis permitted to pass through the opening in the end wall 3l, and theother part being the outer part of the electron stream, which isarrested by the end wall 3l and thus prevented from reaching thecollector electrode 33. With the compactness modulation of the electronbeam, the current through the collector electrode 33 increases as thebeam compactness increases and decreases as the `beam compactnessdecreases. rI'he electron current produced by the collection of thearrested electrons upon the end wall 3l varies conversely, increasingwith a decrease of electron beam compactness and decreasing with anincrease of electron beam compactness. It will be readily apparent,therefore, that the electron current due to collection of electrons bythe end wall 3l is modulated in equal amplitude and reversed phase withrespect to the current through the collector electrode. If desired,therefore, a wave analyzer could be coupled to an impedance inserted inseries with the connection of the positive terminal of source 42 to thecylindrical chamber i2. This wave analyzer would yield datacorresponding exactly with the data obtained through use of waveanalyzer 4l connected as shown. Thus, it is optional whether thefrequency distribution of energy due to the compactness modulation ofthe beam be determined by measuring the electron current through apredetermined middle part of the beam or by measuring the electroncurrent outside this middle part of the beam.

It is not essential to the present invention that the electron dischargedevice i i be constructed in the form illustrated in Fig. 1. Varioustypes oi' vacuum tube structures, such as that oi' the Klystron orvelocity modulation tube illustrated in U. S. Patent 2,242,275 vtoRussell H. Varian, could be employed as the electron discharge device llthe conduit I5 then being replaced by the tubular connection throughwhich the tube is connected to a vacuum pump during manufacture. Sincemany types of vacuum tubes of the above class incorporate electron beamprojecting devices as well as electron collector electrodes, such vacuumtubes are inherently suited to be connected in the same manner as theelectron discharge device Il in Fig. 1, so that no auxiliary electrondischarge device need be employed for measurement of gas pressure in theKlystron. Certain other types of vacuum' tubes, also employing directedelectron beams, are adapted to be operated while in the evacuationprocess, and to give output vs. frequency data indicating their own gaspressures.

Since many changes could be made in the above construction and manyapparently widely different-embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanyingdrawingshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Gas pressure responsive apparatus comprising: an electron dischargedevice having a vchamber including an electron entrance and an electronexit therein, means for directing a concentrated-stream of electronsthrough said electron entrance toward said electron exit, and acollector electrode positioned beyond said exit for receiving electronspassing therethrough; means including a load impedance connected betweensaid collector electrode and said electron stream directing means forproviding a current flow path; and means coupled to said load impedancefor determining the variation of amplitude with frequency of alternatingcomponents of current through said collector electrode.

2. Apparatus as defined in claim 1, wherein said means including a loadimpedance for providing a current iiow path also includes means forraising the potential of said collector electrode above the potential ofsaid chamber.

3. Apparatus for determining the gas presm sure within an evacuatedspace, lcomprising means for projecting a beam of electrons through saidspace, a collector electrode positioned in the path of said electronbeam for receiving said electrons, circuit means coupling said collectorelectrode to said projecting means for conducting current therebetween,and frequency-responsive means coupled to said collector electrode andsaid beam projecting means for determining the frequency distribution ofvariations of current in said electrode.

4. Apparatus as defined in claim 3, further including means'positionedadjacent said collector electrode and between said projecting means andsaid collector electrode for arresting electrons which are divergentfrom said path.

5. High-vacuum gas pressure measuring apparatus comprising means forprojecting a beam of electrons through an evacuated space, means forselectively rejecting electrons beyond a central portion of said beamand receiving electrons in said centralportion of said beam, and

frequency-selectivo amplitude-responsive means coupled to saidlast-named means and said beam projecting means for analyzing thefrequency distribution of voltage variations of said electrode todetermine the gas pressure in said space.

6. High-vacuum' gas pressure measuring apparatus comprising means forprojecting a beam asuma of electrons through an evacuated space, means ffor separately receiving electrons within a central portion of said beamand those electrons in the outer portion of said beam, andfrequencyselective amplitude-responsive means coupled to said last-namedmeans and said beam projecting means for analyzing the frequencydistribution' 8. The method of determining the gas pressure within anevacuated chamber, comprising pro- Jecting a beam of electrons along anaxis in said chamber, arresting those electrons which are separated fromsaid axis by more than a predetermined distance. and measuring thefrequency distribution of alternating components of variation of thenon-arrested electrons.

9. The method of measuring gas pressure within an evacuated spacecomprising directing a beam of electrons through said space andproducing ions therein at a rate determined by the gas pressure insaid-space, the compactness of said beam being modulated as said ionsare produced, and measuring the alternating energy versus frequencycharacteristics of the electrons in a predetermined portion of saidbeam.

RUSSELL H. VARIAN.

REFERENCES ITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,649,016 Buckley Nov. 15, 19272,081,429 Gaede May 25, 1937 2,242,275 Varian May 20, 1941 Cert-iicateof Correction March 28, 1950 Patent No. 2,501,702

RUSSELL H. VARAN 1t is hereby certified that error appears in the printnumbered patent requiring correction as follows'.

Column l, line 27, before the syllable elec insert the Words electronbeam becomes as great as the number of;

ers Patent should be read with this correction the Patent Ofliee.

and that the said Lett same may eonorm to the record of the case 1n dsealed this 20th day of June, A. D. 1950.

Signed an ed speecation of the above therein that the [SEAL] THOMAS F.MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,501,702 March 28, 1950 RUSSELL H.VARIAN It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows:

Column l, line 27, before the syllable elec insert the Words electronbeam becomes as great afs the number of;

nt should be read With this correction therein that the and that thesaid Letters Pate d of the case in the Patent Office.

same may conform to the recor Signed and sealed this 20th day of June,A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Asse'stfm Uommz'ssz'meer of Patents.

